Defending the United Kingdom against invasive, non-native species costs as much as £1.7 billion each year, according to a report by the BBC Countryfile programme.

The American mink has contributed to a rapid decline in the native water vole in the UK

Threat to native wildlife

The programme highlights the threats that so-called ‘alien’ species are posing to the UK’s native animals and plants, and the high cost of tackling the problem.

Invasive, non-native species are those which have been introduced by humans to areas outside of their natural range and, in the absence of natural predators and parasites, increase and spread to the detriment of native species and human interests. Worldwide, invasive species are considered to be the largest cause of biodiversity loss after humans.

The signal crayfish is a voracious predator and has a negative impact on native crayfish species in Europe

Alien invaders

An example of a problematic invasive species in the UK is the zebra mussel, which arrived in the UK around ten years ago on the hulls of ships from Europe. This species reproduces so rapidly that it smothers native mussels and can block water systems. At Rutland Water in Leicestershire, England, the local water company has to spend around half a million pounds a year dealing with the species, and has installed filters to the reservoir to prevent the zebra mussels from spreading through and choking their water pipes.

Introduced to the UK from North America, the grey squirrel has caused a decline in native red squirrels

Foreign plants are also causing major problems. Japanese knotweed alone is estimated to cost the UK economy £165 million each year to eradicate, and mortgages are being turned down due to the damage this species can do to properties. Of the £1.7 billion spent each year on dealing with invasive species in the UK, around £3 million goes towards clearing non-native weeds which can block canals, rivers and ponds.

Threats to health

As well as threatening native species and having an economic impact, invasive animals and plants can also pose risks to human health. For example, the sap of giant hogweed can cause painful blisters, while the hairy caterpillars of oak processionary moths can trigger allergic reactions.

According to BBC Countryfile’s investigations reporter, Tom Heap, “From the grey squirrel, American crayfish, mink driving water voles from our river banks and the small but scarily named ‘killer shrimp’, a whole host of animals and plants are playing their part in colonising our countryside. Luckily, of the 2,000 non-native species living among us, only a few hundred are actually harmful.”